Recently, in many product categories, the use of system LSI having more densely packed and more highly integrated multifunctional ICs, i.e. nanotechnology ultra-VLSI devices (hereinafter simply referred to as “new semiconductor devices”) has increased. Thus, the application range thereof is diverse, from digital appliance daily necessities such as portable phones to space gadgets. Thus, to sustain modern society, new semiconductor devices are indispensable components.
On the other hand, one of the causes of restricting performance of recent highly-packed and highly-integrated electronic devices that feature ultra-low power consumption, ultra-high speed operation and ultra-compact size is a problem associated with the material's strength, i.e., delamination of circuit pattern breakage of a circuit disconnection package, a subtle deformation of the semiconductor wafer, or the like, which occurs when manufactured or used. Accordingly, it is desired to establish an evaluation technique to overcome such problems associated with the material's strength that occur when manufacturing or using a device.
At the same time, there has been a serious need for more efficient production of new semiconductor devices, i.e., higher yields. A factor that prevents the need from being satisfied is the quality of single crystal wafers which is the material of the new semiconductor devices. The quality of the single crystal wafer is evaluated by the perfection of the arrangement of atoms which constitutes the wafer. A current severe problem which causes disarray of the arrangement is the residual stress of the wafer.
A process for producing device chips from a wafer often uses elevated temperatures of about 700° C. to about 1000° C. At this time, because of the high temperature, the critical resolved shear stress (σ CRSS) significantly drops. Then, if the superimposition of the thermal and residual stresses exceeds σ CRSS, the single crystal would induce crystal gliding or translation gliding. Therefore, it is found that suppressing the residual stress of a wafer as low as possible is necessary.
In a typical process of producing semiconductor wafers, particularly silicon (herein after referred to as “Si”) wafers often obtain a residual stress during the lapping process and the polishing process. Since considerable effort is put into slicing an elongated cylindrical (columnar) ingot into disk-shaped wafers, the polishing amount when polishing a sliced surface is controlled as to be as small as possible. Conversely, it is known that an insufficient polishing amount causes the occurrence of and an increase in the residual stress.
Semiconductor wafers are produced and processed in a clean room. Thus, it is desirable that the residual stress is measured in a non-contact manner. Therefore, photoelastic measurement techniques have been conventionally used for measuring the residual stress of semiconductor wafers.
At the earliest stage of the history of photoelastic measurement, the residual stress upon a sample was measured using the interference pattern produced when light is impinged upon (transmitted through) a semiconductor wafer sample which was sliced as thick as 10 mm. In this case, however, the sample wafer was so thick that the sample wafer could only be used for measuring the residual stress and could not be returned to the production line. Thus, the sample wafer was wasted. For this reason, a 100-percent inspection was impossible.
Conventional photoelastic measurement methods using laser beams are roughly classified into two groups: methods using a photoelastic fringe pattern (fringe analyses); and methods not using a photoelastic fringe pattern (sub-fringe analyses).
The fringe analysis is suitable for roughly grasping an overview of stress distributed over a sample, wherein the stress distribution of the sample is obtained in an experimental analysis fashion from two fringe patterns: an isoclinic fringe pattern given by a linear polariscope (principal stress direction distribution chart); an isochromatic fringe pattern given by a circular polariscope (principal stress difference distribution chart). Generally, this method is not suitable for measuring stress distributed over a very small area or measuring subtle stress.
One technique for measuring stress distribution of a semiconductor ingot or a semiconductor wafer with this fringe analysis is as follows.
Lederhandler measured according to the fringe analysis of residual stress distribution of an Si ingot which had been grown by the Czochralski technique (CZ technique, one of the crystal pulling methods) and pointed out that the temperature gradient during crystal growth exceeds the yield stress of Si.
S. R. Lederhandler, “Infrared Studies of Birefringence in Silicon,” J. Appl. Phys., 30-11 (1959), 16311638.
Other techniques for measuring the stress distribution of a semiconductor ingot or a semiconductor wafer with this fringe analysis method are listed below.
K. Date, “Stress Measurement with High Sensitivity in Wafers Using Infrared Photoelasticity,” Proc. of Advanced in Elec. Pack., Vol. 2 (1992), 985-989.
R. O. Denicola and R. N. Tauber, “Effect of Growth Parameters on the Residual Stress and Dislocation Density of Czochralski-Grown Silicon Crystal,” J. Appl. Phys., 42-11 (1971), 4262-4270.
P. Dobrilla and J. S. Blakemore, “Optical mapping of residual stress in Czochralski grown GaAs,” Appl. Phys. Lett., 48(19) (1986), 1303-1305.
G. Qin, H. Liang, S. Zhao and H. Yin, “Measurement of Stresses in Silicon Wafers with the Infrared Photoelastic Method,” Chin. J. Infrared and Millimeter Waves, 7(2) (1987), 139-144.
M. Yamada, M. Fukuzawa, N. Kimura, K. Kaminaka and M. Yokogawa, “Quantitative photoelastic characterization of residual strain and its correlation with dislocation density profile in semi-insulating LEC-grown GaAs wafers,” Proc. 7th Conf. on Semi-insulating III-V Materials, Ixtapa, Mexico, (1992), 201210.
On the other hand, the method not using a photoelastic fringe pattern is effective when a photoelastic fringe pattern is not observed and this method is suitable for measuring stress distributed over a very small area or measuring subtle stress. This method is used in the case when a fringe cannot be observed since the stress is subtle or when the precise stress between fringes is measured.
Specifically, a birefringence amount is determined from the difference between laser polarization before incident upon a sample and laser polarization after being transmitted through the sample, and then converted into stress. Thus, the mean stress within a laser spot area is measured. Accordingly, in order to measure stress distributed over a very small area, a point-by-point measurement should be carried out by the use of a laser having a small laser spot diameter.
For example, when it is desired to obtain an overview of the stress distribution of one semiconductor wafer, it is necessary to deliver the wafer by an X-Y stage, measure stress states at a plurality of points, and obtain the overview there from.
Techniques for measuring subtle-stress distribution are as follows. Clayton et al. developed a scanning birefringence mapping system, which requires rotation of a sample during measurement of its stress, for measuring the residual stress of the LEC-grown GaAs wafer.
R. D. Clayton, I. C. Bassignana, D. A. Macquistan and C. J. Miner, “Scanning birefringence mapping of semi-insulating GaAs wafers,” Semi-insulating III-V Materials, Ixtapa, Mexico, (1992), 211216
Yamada developed a computer-controlled infrared polariscope, which requires rotation of two optical elements (a polarizer 5 and an analyzer 10) during measurement of stress, for measuring the residual stress of the LEC-grown GaAs wafer (see, for example, non-patent reference 8).
M. Yamada, “High-sensitivity computer-controlled infrared polariscope,” Rev. Sci. Instrum., 647 (1993), 1815-1821.
Liang et al. developed a linear polariscope which requires always rotating an analyzer 10 and also making reference to the possibility of measuring the residual stress of an Si wafer (see, for example, non-patent reference 9).
H. Liang, S. Zhao and K. Chin, “A new method of determining the stress state in microelectronic materials,” Meas. Sci Technol., 7 (1996), 102-105.
Especially, the aforementioned Liang et al. experimentally clarified the residual stress of the Si wafer by the use of photoelastic measurement technology. Liang et al. quantitatively researched the residual stresses induced during the polishing process and the like, and reported that the stress of some MPa remains in terms of the principal stress difference.
Currently, in Japan and Germany, SIRDs are available from TePla AG Jena Office (the Japan agency: AIMEK) as devices for measuring the residual stress of Si wafers.
For conventional fringe measurement devices, when a subtle stress is measured, the stress measurement becomes difficult in inverse proportion to the thickness of a test piece. Therefore, there have been few cases of success for measuring the residual stress of semiconductor wafers having a thickness of about 600 μm. Generally, the thickness limit for measuring the stress of a plate-like crystal is a few millimeters.
Furthermore, in the devices of Liange and Yamada, the principal axis of the analyzer is a straight line. They are sub-fringe measurement devices of the type that rotates the sample or an analyzer, i.e., optical elements, for detecting the direction of stress.
When the linearly polarized light is passed through a test piece, the direction and magnitude of the stress of the test piece can be explored by the following method, and the direction of the stress of the test piece is detected when the angle between the direction of the stress and the linear polarization light is 0 and 90 degrees. On the other hand, the magnitude of the stress can be first detected when the angle between the direction of the stress and the linear polarization light is 45 degrees. Therefore, it is required to rotate the test piece or the entire optical system a half-turn in order to determine such data.
Conventionally, with the photoelastic techniques, when circularly polarized light (perfect circle only) is transmitted through a test piece, the magnitude of stress of the test piece can be determined immediately, but the direction thereof cannot be known. This is because, for the circularly polarized light, the direction of the stress exerting upon the test piece does not have meaning but only the magnitude thereof has meaning.
Thus, it is considered that further enhancement of the accuracy and speed of the stress measurement is difficult so long as a mechanical movement such as rotation is involved.
These devices suffer from disadvantages that there is no hope for enhancing the measurement speed and accuracy because of rotation, and that they are not suitable for being subjected to inline measurement which requires incorporating such a birefringence phase difference measurement device into the production line.
The SIRD device of the TePla AG Jena Office does not indicate a value of the absolute value of the residual stress (gram number per square millimeters). It detects only a relative value. Accordingly, it is inconvenient for being used for quality control in the production process.
Thus, there is no example of measuring the stress distribution of semiconductor wafers by a sub-fringe measurement device of a type without rotating optical elements.
While there has been a report that a unique photoelastic experimental device was experimentally produced and the stress measurement of semiconductor wafers as thick as 600 μm was successful, such a device was merely an experimental device and could not stably ensure high measuring power.